The present invention relates to a method for the recovery of the elemental components of solid waste materials containing group III-V elements and group V elements which are generated during the production of III-V semiconductors.
Semiconductor devices formed from group III elements and group V elements, such as, for example, gallium arsenide (GaAs), gallium phosphide (GaP) and indium phosphide (InP), are used for a multitude of military and industrial applications, such as lasers, light-emitting diodes and communications devices, in the United States and throughout the world. Manufacturing processes devoted to the fabrication of these devices generate large volumes of waste materials which contain valuable gallium and indium. Low-cost procedures devoted to the recovery of these metals are economically advantageous to the manufacturer of group III-V materials.
Manufacturers grow bulk crystals of group III-V materials in large boules or ingots. The manufacturer performs a number of processing steps wherein the boules are examined for their crystallographic orientation and then cut and slabbed into wafers. This cutting and slabbing operation produces a large volume of solid wastes, which range in size from micron-sized fine particles to boule ends and large crystal pieces. Conventional sawing operations utilize a single revolving metal blade impregnated with diamond or other hard abrasive; newer and more time-efficient designs utilize numerous abrasive-coated wires which run back and forth simultaneously over the crystal surface. The cutting and slabbing process generally results in two waste streams--fines and small particles that are carried out with the cutting lubricant, and larger pieces that remain behind. Fines are typically removed from the oil or water lubricant either through centrifugation or a settling process; larger pieces are hand sorted for either disposal or reuse. The decision to recycle or discard larger pieces often depends upon whether the original boule was doped or undoped with other chemical elements (e.g. silicon), or how "contaminated" it might be with other materials from the sawing operation (such as abrasive, saw mount material, etc.)
Due to the economic value of gallium, a number of recovery methodologies have been developed and tested over the years to recover gallium from gallium arsenide waste materials. The first step in these processes is typically to separate the gallium and arsenic. A known method to accomplish this separation is to contact the solid waste with a large volume of an aqueous solution or a heated bath, allowing a chemical reaction to facilitate separation. These separation media typically consist of oxidizing species such as hydrogen peroxide, nitric acid, or sodium hydroxide. Once in solution or in the heated bath, the gallium is then sequentially removed using a number of additional methodologies. While these practices have been demonstrated as workable, they involve the introduction of a larger-volume medium to effect separation. This larger-volume medium must then itself be treated (e.g., to preclude the release of toxic arsenic and/or to recover gallium). Thus, rather than separating the constituent elements from one another, these approaches require further processing for metal recovery from a much larger waste stream.
Thermal processing of GaAs solid wastes to recover gallium has also been demonstrated in the past. While thermal separation under air has been achieved for GaAs, this results in the formation of arsenic and gallium oxides. For material recovery, these oxide "slags" require an additional processing step (reduction) to obtain reusable metals. Therefore, separating under an inert atmosphere or under vacuum is desirable in order to minimize the number of processing steps and subsequently the overall cost of the recovery operation. Separating under an inert atmosphere or vacuum has been attempted, and many of the processes require particular conditions for thermal separation. However, recovery and purification processes for group V elements have not been correspondingly developed. For example, U.S. Pat. No. 4,812,167 to Inooka teaches that fine GaAs particles are first "calcined" at 400.degree. C. in order to coke binding oils, which prevents fine materials from being removed under vacuum. Separation of gallium and arsenic is then achieved at a temperature range of between 1050.degree. C. and 1150.degree. C. and a pressure from 10.sup.-4 to 10.sup.-1 Torr. The liquid gallium is then purified by heating it to between 1000.degree. C. and 1500.degree. C. (at ambient pressure) and removing the surface layer. However, no recovery or purification of arsenic is attempted in that process. This is the case in all other reported thermal separations--liquid gallium is recovered, but condensed arsenic is disposed of, presumably due to its low cost.
As another example, U.S. Pat. No. 4,362,560 to Abrjutin et al. teaches a vacuum thermal separation process for the recovery of gallium from gallium arsenide semiconductor wastes. The process involves vacuum thermal separation of the waste, sublimation of arsenic and condensation thereof, cooling of the gallium melt, filtration of the gallium melt and a hydro-chemical treatment of the gallium melt with nitric acid or sulfuric acid, followed by purifying crystallization of the gallium melt after the hydro-chemical treatment. All of the steps of the Abrjutin et al. process are conducted at or above the melting point of gallium. This process also introduces another liquid into the recovery process, the hydro-chemical treatment fluid, and further fails to recover the arsenic. Additionally, the washing of the gallium melt with nitric acid and/or sulfuric acid is a difficult process to successfully carry out.
Group III-V material semiconductor waste materials range in size from entire boules to fine dust. Typically, current methodologies only recycle the very largest pieces. However, the smaller particles represent nearly 50% of the produced wastes making their recovery desirable. Under vacuum conditions, however, the smaller size pieces would be carried toward the vacuum pump due to their smaller mass and, for that reason, recovery entails coalescing these smaller particles into a mass with sufficient weight to resist transport under vacuum. U.S. Pat. No. 4,812,167 to Inooka describes a process in which binding oils are "calcined" at 400.degree. C. to bind smaller particles together. However, such a process is undesirable for binding the particles because it introduces a significant measure of carbon contamination into the material to be separated. The additional carbon affects the electronic performance of components fabricated from the recycled materials.
One problem with current disposal methodologies is that GaAs, GaP and InP may be converted to toxins, such as arsine (AsH.sub.3) and phosphine (PH.sub.3), after exposure to acidic concentrations. Arsine gas is lethal in concentrations as low as 250 ppm while lower concentrations result in chronic effects. Phosphine exhibits a comparable toxicity. Landfills are typically anaerobic (reducing) environments, which simultaneously form organic acids such as acetic and formic acids. GaAs, GaP and InP exposed to typical landfill conditions can be easily converted to arsine and phosphine gases. Even if released, arsenic is oxidized to the trivalent or pentavalent state and aqueous-phase arsenic still represents a measurable toxic environmental threat.
Another problem with existing recycling approaches is that only the metal gallium is being recovered, purified, and recycled back into the overall semiconductor manufacturing process. On a material weight basis, approximately 90% of solid GaAs wastes are currently disposed of, and 10% are recycled. Typically, only undoped pieces having thicknesses greater than 0.25 inch (0.63 cm) are recycled. Arsenic is not presently being recovered from GaAs because arsenic is very inexpensive from a raw materials cost standpoint, and it is, therefore, less expensive to simply purchase virgin raw arsenic for crystal growth. Indium, from InP semiconductor production, is not presently being recycled because the process utilized to recover GaAs, when applied to InP, results in the formation of elemental phosphorous, which is a significant fire hazard. For this reason, recyclers refuse to accept InP wastes and, for similar reasons, GaP.
Although the prevailing attitude within the group III-V material bulk crystal manufacturing industry maintains that the existing disposal and recycling approaches are satisfactory, no methodologies have been developed to recover indium, phosphorus or arsenic. Under existing U.S. environmental laws, the original crystal manufacturer is liable for any future environmental cleanup costs relating to environmental hazards caused by the crystal materials that it produces, with or without recycling of the gallium from the waste stream. Perhaps the only reason why such costs have not begun to be incurred by today's crystal manufacturers is that the industry is still in its infancy, and so environmental damage that is directly attributable to III-V semiconductor manufacture has not yet been observed. Judging from the large monetary sums currently being awarded to localities (for liabilities) and environmental contractors (for cleanup) for, for example, disposal of arsenic-containing wood preservatives, it is safe to predict that the future holds some very unpleasant economic surprises for today's crystal manufacturers if approaches are not altered to allow for recovery and reuse group III elements other than gallium and group V elements.
Recycling of both the group III element and the group V element from III-V manufacturing wastes back into the crystal-growing operation yields both short-term and long-term economic benefits. For these reasons, a need currently exists in the art for a method and apparatus which can be used to recover the elemental components of group III-V semiconductor solid waste materials so that they can be recycled into the crystal growing process.